Plasma density information measuring method, probe used for measuring plasma density information, and plasma density information measuring apparatus

ABSTRACT

A plasma density information measuring method capable of easily measuring the plasma density information over the long term, a probe for measuring the plasma density information, and a plasma density information measuring apparatus are disclosed. A measuring probe is set such that a tip end of a glass tube of the measuring probe is brought into contact with plasma PM to be measured. High-frequency power sent through a coaxial cable is supplied to the plasma PM from a loop antenna through a wall of the tube, and reflection power of the high-frequency power is received by the loop antenna to obtain a counter frequency variation of reflection coefficient of the high-frequency power. In the obtained reflection coefficient, a portion thereof in which the reflection coefficient is largely reduced is a peak at which strong absorption of high-frequency power is caused due to the plasma density. The plasma density can be obtained from the plasma absorption frequency.

This is a Division of application Ser. No. 09/357,773 filed Jul. 21,1999. The disclosure of the prior application(s) is hereby incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to: a plasma density information measuringmethod in plasma used in a producing process of a thin film component, aparticle beam source or an analysis apparatus; a probe for measuring theplasma density information used for measuring the plasma densityinformation; and a plasma density information measuring apparatus; andmore particularly, to a technique for easily measuring the plasmadensity information over the long term.

(2) Description of the Related Art

In recent years, the use of plasma is increased. In a producing processof a thin film component, using high-frequency plasma generated byhigh-frequency power (high-frequency electric power) in a range from aRF frequency band of about 10 MHz to a micro frequency band of 2.45 GHz,etching process or CVD (chemical-vapor deposition) are conducted. Insuch a plasma application technique, it is extremely important forconducting appropriate process to sufficiently grasp the informationconcerning plasma density which excellently shows the characteristics ofgenerated plasma. In the case of typical plasma comprising monovalentpositive ion and electron, the positive ion density and the electrondensity are substantially equal to each other due to the propertiesparticular to plasma that electrically neutral state is maintained, theelectron density is generally called as plasma density.

Conventionally, as a method for measuring the electron density inplasma, there is an electron beam irradiation type plasma vibrationprobe which was developed relatively recently, in addition to aLangmuir/probe method and a microwave interference measuring method.

The Langmuir/probe method is a method in which a metal probe is directlyexposed in plasma in this state, direct current bias voltage, or directcurrent bias voltage on which high-frequency voltage is superposed isapplied to the metal probe, and based on the current value flowingthrough at that time, electron density is measured.

The microwave interference measuring method is a method in which achamber for generating plasma is provided with windows which are opposedto each other with plasma positioned therebetween, microwave (e.g.,single color laser light) is radiated to the plasma through one of thewindows, and the microwave ejected from the other window is detected,and electron density is obtained based on phase contrast between theradiated microwave and ejected microwave.

The electron beam irradiation type plasma vibration method is a methodin which a hot filament is placed in a chamber, and based on frequencyof plasma oscillations generated when electron beam is irradiated to theplasma from the hot filament, electron density is obtained.

However, when the Langmuir/probe method is used for reactive plasma,there is a problem that the measuring can not be continued for a longtime (i.e., life time is short). This is because that stains comprisinginsulative films are adhered on a measured metal probe within a shorttime, the current value flowing through the metal probe is varied, andaccurate measurement can not be continued soon. In order to remove thestains adhered on the metal probe surface, a method in which negativebias voltage is applied to the metal probe to carry out sputter-removingmethod using ion, and a method in which the metal probe is allow to glowto evaporate and remove the stains have been attempted, but the effectis poor, and the problem is not solved by these methods.

Further, the microwave interference measuring method has a problem thatthe measurement can not be conducted easily. This is because alarge-scale and expensive apparatus and adjustment of microwavetransmission path are necessary, the phase contrast between the radiatedmicrowave and ejected microwave is small and thus, it is difficult tomeasure precisely. Further, in the case of the microwave interferencemeasuring method, there are drawbacks that only the average density canbe obtained, there is no spatial resolution.

Furthermore, in the case of the electron beam irradiation type plasmavibration probe method, in addition to anxiety of plasma atmospherecontamination due to tungsten which is evaporated from the hot filament,there is a problem of anxiety of interruption of measurement caused bybreak of hot filament. Especially in the case of plasma using oxygen orchlorofluorocarbons gas, the hot filament is easily cut or broken, andit is necessary to frequently exchange the filament, it can not be saidthat this is practical.

SUMMARY OF THE INVENTION

In view of the above circumstances, it is an object of the presentinvention to provide a plasma density information measuring methodcapable of easily measuring the plasma density information over the longterm, a probe for measuring the plasma density information, and a plasmadensity information measuring apparatus.

To achieve the above object, the present invention provides thefollowing structure.

That is, a plasma density information measuring method of the presentinvention comprises the steps of:

supplying high-frequency power to plasma;

measuring a physical amount indicative of reflection or absorption stateof the high-frequency power by plasma load; and

obtaining a frequency at which strong high-frequency power absorption iscaused due to plasma density, i.e., a plasma absorption frequency basedon the measurement result of the physical amount.

In the case of the plasma density information measuring method of thepresent invention, the high-frequency power is supplied to plasma, thephysical amount indicative of reflection or absorption state of thehigh-frequency power by plasma load is measured (for example, thereflection amount of the high-frequency power or impedance value of theplasma load is measured). Based on the measured result of the physicalamount, plasma absorption frequency at which high-frequency powerresonant strong absorption is generated due to plasma density isobtained. If the high-frequency power resonant strong absorption iscaused, since the physical amount indicative of reflection or absorptionstate of high-frequency power by plasma load is largely varied, plasmaabsorption frequency can easily be obtained. Since the obtained plasmaabsorption frequency has constant correlation with the plasma density,this is useful plasma density information. In the present invention,high-frequency power, i.e., high-frequency electromagnetic wave issupplied to plasma and thus, even if stains comprising insulative filmsare adhered to the antenna which supplies high-frequency power, there islittle influence, and the plasma absorption frequency can be measuredaccurately.

In this point, the present invention is superior to the conventionalLangmuir probe method. Because in this method, electric current flowingwhen ion in plasma reaches a surface of a metal probe is detected andtherefore, if insulative film is adhered to the metal probe, it isimpossible to measure. Further, according to the present invention,since a hot filament is not used unlike the electron beam irradiationtype plasma vibration probe method, there is no anxiety of breaking offilament, and it is possible to obtain the plasma density informationover the long term.

In the method of the invention, it is preferable that the high-frequencypower is supplied to plasma through a division wall. By interposing thedielectric division wall between the plasma side to be measured and thesupplying side of the high-frequency power, a foreign object should notenter the plasma from the supplying side of the high-frequency power,and plasma can be maintained clean. Further, in the case of reactiveplasma also, the high-frequency power supplying side is not damaged.Furthermore, even if stains such as insulative films are adhered to thesurface of the dielectric division wall, there is no change in themeasuring system, it is possible to obtain the plasma densityinformation for longer time.

In the present invention, for example, the physical amount indicative ofreflection or absorption state of the high-frequency power by plasmaload is measured by measuring an electric current amount of ahigh-frequency amplifier for supplying high-frequency power. Through thehigh-frequency amplifier for supplying high-frequency power, electriccurrent corresponding to a degree of reflection or absorption of thehigh-frequency power by the plasma load flows. Therefore, it is possibleto easily measure the physical amount indicative of reflection orabsorption state of the high-frequency power by measuring this electriccurrent.

In the present invention, for example, the reflection amount ofhigh-frequency power is detected while sweeping high-frequency powerfrequency, and the plasma absorption frequency is obtained based onrelationship between sweep-frequency and a detected result of thereflection amount of high-frequency power. That is, it is possible toeasily obtain a frequency at which the reflection amount of thehigh-frequency power is largely reduced, as a frequency at which thehigh-frequency power resonant strong absorption is caused due to theplasma density, i.e., as a plasma absorption frequency.

In the present invention, a plasma surface wave resonance frequency isobtained as the plasma absorption frequency for example. The surfacewave resonance frequency f is correctly corresponds to the electrondensity n_(e) in plasma.

In the present invention, electron density in plasma to be measured iscalculated in accordance with the obtained plasma surface wave resonancefrequency. That is, the electron density n_(e) in plasma is calculatedin accordance with the surface wave resonance frequency f=ω/2π (whereinω is surface wave resonance frequency). The electron density n_(e) issubstantially equivalent to the plasma density. The electron densityn_(e) can easily be calculated in accordance with the following equation(1):

n _(e)=ε_(o) ·m _(e)·ω_(p) /e ²  (1)

wherein ω_(p): electron plasma angle frequency

[ω_(p)=ω×{square root over ( )}(1+ε)]

ε:dielectric constant of dielectric division wall, ε_(o):vacuumdielectric constant

m_(e): electron mass, e: electron amount

In the present invention, for example, Tonks-Dattner resonance frequencyis obtained as the plasma absorption frequency. If the high-frequencypower is radiated to the plasma, a plurality of absorption spectrum isobserved in addition to the surface wave resonance. It is consideredthat this corresponds to so-called Tonks-Dattner resonance. That is,when electromagnetic wave is radiated from outside of cylindrical plasmaand power absorbed by the plasma is measured, strong absorption iscaused at plurality of frequencies around the electron plasma anglefrequency ω_(p). This phenomenon is cased as Tonks-Dattner resonancefrom the name of the detector. According to subsequent research, it isexplained that the mechanism causing this resonance is that electronplasma wave transmitted in radial direction is excited byelectromagnetic wave, and resonant absorption is caused when the excitedelectron plasma wave is reflected at the plasma end and standing wave isgenerated. Further, since there is relationship between the resonancefrequency and the electron plasma angle frequency ω_(p), if the plasmadensity is varied, the Tonks-Dattner resonance frequency is also varied.That is, the Tonks-Dattner resonance frequency provides plasma densityinformation.

The present invention provides a probe used for measuring plasma densityinformation, comprising:

a dielectric tube whose tip end is closed;

an antenna accommodated in the tube at its tip end side for radiatinghigh-frequency power; and

a cable accommodated in the tube at its rear side and connected to theantenna for transmitting the high-frequency power.

When the plasma density information is measured using the probe used formeasuring plasma density information of the invention, the probe is setsuch that the tip end of the tube is brought into contact with theplasma to be measured, the high-frequency power sent through the cableis supplied to the plasma from the antenna through the dielectric tubewall, and the reflection power of the high-frequency power required formeasuring the plasma absorption frequency is received by the antenna,and taken out through the cable. Since the range where thehigh-frequency power from the antenna influences the plasma is not sowide, it is also possible to obtain a local plasma density informationif the amount of the high-frequency power is adjusted. That is, if theplasma density information measuring probe of the invention is used, itis possible to easily prepare necessary state for measuring the plasmadensity information, and to obtain spatial resolution. Further, sincethe antenna is covered with the dielectric tube, plasma is notcontaminated, and the antenna is not damaged by the plasma and thus, thelifetime is long.

In the probe used for measuring plasma density information of theinvention, it is preferable that the antenna and the cable accommodatedin the dielectric tube are capable of moving along a longitudinaldirection of the tube such that a position of the antenna in the tubecan be varied. In this example, the position of the antenna in thedielectric tube is changed along the longitudinal direction of the tubeseveral times. And plasma absorption frequencies at the antennapositions are measured. Among the several absorption frequenciesobtained by this measurement, the lowest frequency that is not variedeven if the position of the antenna is changed is obtained as a surfacewave resonance frequency.

In the probe used for measuring plasma density information of theinvention, it is preferable that a conductor for preventing a leakage ofejected electromagnetic wave from the antenna is disposed at a positionslightly back from the antenna such as to occlude a gap between thecable and an inner surface of the tube. With this structure, since theconductor disposed slightly back from the antenna prevents theelectromagnetic wave power discharged from the antenna from leakingoutside except plasma, measuring error due to the leakage of thehigh-frequency power is avoided.

In the probe used for measuring plasma density information of theinvention, it is preferable that probe cooling means for forciblycooling the probe is disposed. According to this example, since theprobe is forcibly cooled by the probe cooling means, the measuring errorby temperature rise of the tube or cable is avoided.

In the probe used for measuring plasma density information of theinvention, it is preferable that the cable for transmittinghigh-frequency power comprises a conductor tube for a core wire and ashield, and an insulative ceramics material for filling a gap betweenthe core wire and the conductor tube. According to this example, sincethe gap between the core wire and the conductor tube. According to thisexample is filled with the heat-resistant insulative ceramics material,the heat-resistance of the cable is enhanced.

In the probe used for measuring plasma density information of theinvention, it is preferable that a surface of the dielectric tube iscoated with metal such that a measuring area of the dielectric tube isnot coated. According to this example, since the surface of thedielectric tube is coated with metal such that the measuring area of thedielectric tube is not coated, the local state of the measuring areathat is not coated with metal is strongly reflected to the measuredresult, and the spatial resolution is enhanced.

In the probe used for measuring plasma density information of theinvention, it is preferable that the antenna is extended closely alongan inner surface of the dielectric tube. With this structure, since thehigh-frequency power irradiated from the antenna is effectively suppliedto the plasma, the supply amount of the high-frequency power may besmall, and the measuring precision is enhanced.

A plasma density information measuring apparatus of the presentinvention comprises:

sweep-frequency type high-frequency power supplying means for supplyinghigh-frequency power to plasma while sweeping frequency;

reflection power amount detecting means for detecting a reflectionamount of the high-frequency power; and

power reflection coefficient frequency characteristics obtaining meansfor obtaining a counter frequency variation of reflection coefficient ofhigh-frequency power based on a sweep-frequency of the high-frequencypower and the detected result of the reflection amount of high-frequencypower.

According to the apparatus of the invention, it is possible to easilymeasure the plasma absorption frequency as the plasma densityinformation.

In the apparatus of the invention, it is preferable that the apparatusfurther includes a dielectric division wall interposed between plasmaand the sweep-frequency type high-frequency power supplying means.According to this structure, since the dielectric division wallinterposed between plasma and the sweep-frequency type high-frequencypower supplying means is provided, it is possible to maintain the plasmaclean.

In the apparatus of the invention, it is preferable that the apparatusincludes the above-described plasma density information measuring probe,and high-frequency power is supplied from the antenna in the tube toplasma using a tube wall of the dielectric tube as a division wall, aplurality of antennas are accommodated in the dielectric tube such thatdistances between a tip end of the tube and the antennas are differentfrom one another, and the power reflection coefficient frequencycharacteristics obtaining means obtains a counter frequency variation ofreflection coefficient of high-frequency power for each of the antennas,and a plasma absorption frequency appearing at the same frequency in thecounter frequency variations is obtained as a plasma surface waveresonance frequency. With this structure, it is possible to easilymeasure the plasma density information, and to generate the spatialresolution. In addition, it is possible to easily obtain the plasmasurface wave resonance frequency from the counter frequency variation ofreflection coefficient of the high-frequency power from the antennashaving different distances from the tip and of the tube.

In the apparatus of the invention, it is preferable that a plasmadensity information measuring probe is inserted in a chamber whichgenerates plasma for forward and backward movement, and the probe ismoved such that a tip end of the probe is pulled backward from ameasuring position in the chamber to a retreat position in the vicinityof a wall surface of the chamber when measurement is not carried out.With this structure, since the probe is moved such that a tip end of theprobe is pulled backward from a measuring position in the chamber to aretreat position in the vicinity of a wall surface of the chamber whenmeasurement is not carried out, even if the plasma allows stains toadhered to the surface of the probe, it is possible to move the probetoward the plasma only when the measurement is carried out, to preventthe probe from being contaminated, and to keep using the probe for along time.

In the apparatus of the invention, it is preferable that protectingmeans for blocking excessive plasma generating high-frequency powerwhich enters the antenna in the probe is provided behind the plasmadensity information measuring probe. With this structure, when excessiveplasma generating high-frequency power enters the antenna in the probe,the protecting means provided behind prevent the excessivehigh-frequency power, thereby preventing the apparatus from beingdestroyed. Especially when the generated plasma disappears unexpectedly,there is an adverse possibility that the high-frequency power forgenerating the plasma is directly placed on the antenna, and the probecontrol section is destroyed. However, this adverse possibility isovercome by the protecting means.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in thedrawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentalities shown.

FIG. 1 is a block diagram showing a plasma processing system accordingto an embodiment of the present invention;

FIG. 2 is a vertical sectional view showing a measuring probe used inthe system of the embodiment;

FIG. 3 is a transverse sectional view showing the measuring probe usedin the system of the embodiment;

FIG. 4 is an equivalent circuit diagram of a directional coupler used ina plasma density information measuring apparatus;

FIG. 5 is a sectional view showing a position changing state of a loopantenna in a measuring probe;

FIG. 6 is a graph showing reflection coefficient frequencycharacteristics of high-frequency power for measuring the plasma densityinformation;

FIG. 7 is a graph showing the relationship between plasma absorptionfrequency and length of tip end of a tube of the measuring probe;

FIG. 8 is a vertical sectional view showing a modification of themeasuring probe;

FIG. 9 is a vertical sectional view showing another modification of themeasuring probe;

FIG. 10 is a block diagram showing a modification of a probe controlsection;

FIG. 11 is a block diagram showing another modification of the probecontrol section;

FIG. 12 is a partial sectional view showing the measuring probe andprobe moving means;

FIG. 13 is a partial vertical sectional view showing a modification of acoaxial cable;

FIG. 14 is a vertical sectional view showing another modification of themeasuring probe; and

FIG. 15 is a vertical sectional view showing another modification of themeasuring probe.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be explained indetail below with reference to the drawings.

FIG. 1 is a block diagram showing a plasma processing system capable ofcarrying out one example of a plasma density information measuringmethod of the invention comprising one example of a plasma densityinformation measuring probe and one example of a measuring apparatus ofthe invention. FIG. 2 is a vertical sectional view showing one exampleof structure of the plasma density information measuring probe (whichwill be omitted as “measuring probe” hereinafter) according to theinvention. FIG. 3 is a transverse sectional view showing a structure ofone example of the measuring probe of the invention.

As shown in FIG. 1, the plasma processing system of the embodimentcomprises a cylindrical stainless steel chamber 1 of about 10 cmdiameter having a space S in which reactive plasma (which will beomitted as “plasma” hereinafter) PM is generated, an ignition electrode(ignition antenna) 2 disposed in the chamber 1 for generating plasma, avacuum discharge pump 4 which is in communication with the space S ofthe chamber 1 through a discharge pipe 3, and a gas source 7 which is incommunication with the space S of the chamber 1 through a gas supplypipe 6 provided therein with a flow rate adjusting valve 5. In addition,a board (not shown) of a work (object to be processed) W, and atransfer-in/transfer-out mechanism of the work W are also disposed inthe chamber 1 of the system of the present embodiment.

Air in the space S of the chamber 1 is exhausted by the vacuum dischargepump 4 and the space S is kept at appropriate pressure. Atmosphericpressure in the space S when plasma PM is generated is in a range fromsome mTorr to some tens mTorr for example. Gas is supplied from the gassource 7 at appropriate flow rate. Examples of the gas to be suppliedare argon, nitrogen, oxygen gas, fluorine gas, and chlorine gas. The gasflow rate set by the flow rate adjusting valve 5 is in a range from 10to 100 cc/minute for example.

A high-frequency electric source 8 for supplying high-frequency power(high-frequency electric power) for generating plasma is providedoutside the chamber 1. The high-frequency power output from thehigh-frequency electric source 8 is sent to the ignition electrode 2through an impedance matching device 9. The magnitude of thehigh-frequency power output from the high-frequency electric source 8 isin a range from about 1 to about 3 kW for example. The frequency of thehigh-frequency power should not be limited to particular frequency, butthe frequency of the high-frequency power is usually in a range from RFband which is typically 13.56 MHz to a microwave band of about 900 MHzto 2.45 GHz.

In the case of plasma of inductively coupled RF discharge plasma, aninduction coil is used as the ignition electrode 2, and in the case ofcapacitive coupled RF discharge plasma, a flat electrode is used as theignition electrode 2. Further, in the case of microwave discharge plasmain which frequency of high-frequency power is frequency of microwaveband, a horn antenna, a slot antenna or an opened waveguide is used asthe ignition electrode 2.

When frequency of high-frequency power is RF band frequency, a matchingcircuit in which inductance and capacitance are combined is used as theimpedance matching device 9. When frequency of high-frequency power ismicrowave band frequency, an EH tuner or a stub tuner is used as theimpedance matching device 9.

In the case of the system of the present embodiment, there is provided areflection power monitor 10 which detects a reflection amount ofhigh-frequency power returning to the electric source side without beingabsorbed by plasma load, and which sends the detected reflection amountof the high-frequency power to the power adjusting section 11. The poweradjusting section 11 controls the impedance matching device 9 such thatthe reflection amount (reflection electric power amount) ofhigh-frequency power becomes minimum, and stabilizes the plasma density.

The work W is subjected to etching process or the CVD (chemical-vapordeposition) by plasma PM generated in this manner. The system of theembodiment is provided with an apparatus for measuring informationconcerning the plasma density which excellently shows thecharacteristics of the plasma PM. In order to subject the work W toappropriate processing, it is very important to measure the plasmadensity information to grasp the characteristics of the plasma PM.

As shown in FIG. 1, the plasma density information measuring apparatusof the embodiment comprises a measuring probe 12 mounted to a wall ofthe chamber 1, and a probe control section 13 disposed outside thechamber 1. A specific structure of the measuring probe 12 will beexplained first.

As shown in FIGS. 2 and 3, the measuring probe 12 comprises a dielectrictube 14 whose tip end is closed and rear end is opened into atmosphere(outside air), a loop antenna 15 for radiating high-frequency power, acoaxial cable 16 connected to the loop antenna 15 for transmitting thehigh-frequency power to the loop antenna 15, and an aluminum conductorpiece 17 for preventing leakage of radiated electromagnetic wave. Thedielectric material forming the tube 14 is not particularly limited, butappropriate examples thereof are reinforced heat-resistant glass,quartz, and ceramic.

The loop antenna 15 and the coaxial cable 16 are accommodated in thetube 14 such that the loop antenna 15 comes first. The conductor piece17 is disposed at a position slightly rearward of the loop antenna 15such that a gap between the coaxial cable 16 and an inner surface of thetube 14. As a result, measuring error due to leakage of high-frequencypower is avoided.

The measuring probe 12 is inserted and mounted from a through hole 1Aprovided in the wall of the chamber 1 such that a tip end of themeasuring probe 12 is located in the chamber 1. An O-ring 1B isinterposed between an outer peripheral surface of the measuring probe 12and the through hole 1A of the chamber 1 so that vacuum leakage is notcaused by placement of the measuring probe 12.

As shown in FIG. 3, the coaxial cable 16 is of a general coaxialstructure in which fluoroplastic 16 c is interposed between a core wire16 a and a shield wire 16 b continuously surrounding the core wire 16 afrom its outside along the longitudinal direction. Cooling fluid such asair or nitrogen gas is forcibly sent into a gap between the tube 14 andthe coaxial cable 16. As a result, measuring error which may be causeddue to temperature rise of the tube 14 or the coaxial cable 16 isavailable. As sending means of the cooling fluid, the followingstructure can be employed. For example, a thin tube (not shown) isinserted into the gap between the tube 14 and the coaxial cable 16, anda tip end of the thin tube is positioned near the conductor piece 17.The cooling fluid is sent to a deep portion of the tube 14 through thethin tube to cool the measuring probe 12. The cooling fluid is notlimited to gas such as air, and may be liquid such as water.

Further, as shown in FIG. 5, the loop antenna 15, the coaxial cable 16and the conductor piece 17 are moved forward and backward as one unit bypulling or pushing the coaxial cable 16 in the longitudinal direction ofthe tube 14, so that the position of the loop antenna 15 is varied alongthe longitudinal direction of the tube 14. That is, with this measuringprobe 12, the length L between the tip end of the tube 14 and a tip endof the conductor piece 17 including the loop antenna 15 can easily bevaried.

Next, a specific structure of the probe control section 13 will beexplained. The probe control section 13 comprises a swept-frequencyhigh-frequency oscillator 18, a directional coupler 19, an attenuator20, and a filter 21. These elements are connected to the measuring probe12 in the order shown in FIG. 1. The high-frequency oscillator 18outputs high-frequency power for measuring plasma density information ofabout 10 mW at frequency of 100 kHz to 3 GHz while automaticallyconducting swept-frequency. The high-frequency power output from thehigh-frequency oscillator 18 is transmitted to the measuring probe 12through the directional coupler 19, the attenuator 20, and the filter 21in this order.

On the other hand, the high-frequency power for measuring the plasmadensity information is not always emitted from the loop antenna 15 andabsorbed by the plasma load, and some of the high-frequency power is notabsorbed by the plasma load and reflected and returned. The reflectionamount of the high-frequency power which is not absorbed by the plasmaload and returned is detected by the directional coupler 19, and sent toa power reflection coefficient frequency characteristic obtainingsection 22. High-frequency power frequency which is output from thehigh-frequency oscillator 18 is also sent to the power reflectioncoefficient frequency characteristic obtaining section 22.

The filter 21 removes high-frequency power for exciting plasma which ismixed into the probe control section 13 through the antenna 15. Theattenuator 20 adjusts the amount of high-frequency power to be sent tothe measuring probe 12.

As shown in FIG. 4, the directional coupler 19 is of a coaxial structurecomprising a core wire 19 a and a shield wire 19 b continuouslysurrounding the core wire 19 a from its outside along the longitudinaldirection. A short coupling line 19 c is provided along the core wire 19a inside of the shield wire 19 b. A side of the coupling line 19 ccloser to the high-frequency oscillator is grounded through a resistor19 d so that the high-frequency power reflection amount can be detectedat the non-grounded side of the coupling line 19 c.

The power reflection coefficient frequency characteristic obtainingsection 22 obtains counter frequency variation of the high-frequencypower reflection coefficient based on the high-frequency power and thedetected reflection amount thereof, and outputs the detected result to adisplay monitor 23. The counter frequency variation of thehigh-frequency power reflection coefficient is displayed on a screen ofthe display monitor 23 as a graph. That is, in the power reflectioncoefficient frequency characteristic obtaining section 22, expression[detected reflection amount of high-frequency power/total output amountof high-frequency power (constant amount in this embodiment)] iscalculated, and the high-frequency reflection coefficient is obtained,and the obtained reflection coefficient is plotted in correspondencewith frequency which is varied from moment to moment, so that thecounter frequency variation of the high-frequency power reflectioncoefficient is obtained.

When the reflection coefficient is largely reduced, this point is theabsorption peak where strong high-frequency power absorption is causeddue to the plasma density, and this absorption peak is plasma absorptionfrequency. Since there is a constant correlation between the plasmaabsorption frequency and the plasma density, effective plasma densityinformation can be obtained. When the plasma absorption frequency issurface wave resonance frequency, the plasma density information can beobtained by simply calculating electron density n_(e) in plasma which issubstantially equivalent to the plasma density.

Subsequently, a concrete plasma density information measuring example bya plasma density information measuring apparatus of the presentembodiment will be explained.

Atmosphere of the space S in the chamber 1 was adjusted to be argon 10 mTorr. Then, high-frequency power of 13.56 MHz was applied to theignition electrode from the high-frequency electric source 8 at theoutput amount of 1.2 kW, thereby generating reactive plasma PM in thespace S.

The tube 14 of the measuring probe 12 is a Pyrex glass tube having outerdiameter of 6 mm and dielectric constant of 4. The coaxial cable 16 is asemi-rigid cable of 50Ω, and the conductor piece 17 is made of aluminumfoil.

First, as shown in FIG. 2, the measuring probe 12 was set such that thelength L between the base end of the loop antenna 15 and the tip end ofthe tube 14 became 3.5 mm. Then, high-frequency power of 10 mW from 100kHz to 3 GHz was output from the high-frequency oscillator 18 whileconducting swept-frequency. The reflection amount of the high-frequencypower at that time was detected by the directional coupler 19, and thecounter frequency variation of a reflection coefficient ofhigh-frequency power was measured and displayed on the display monitor23 as shown with the uppermost curved line Ra in FIG. 6.

Subsequently, as shown in FIG. 5, the set position of the measuringprobe 12 was changed such that the length L between the base end of theloop antenna 15 and the tip end of the tube 14 became 5.5 mm, 7.5 mm,9.5 mm, 11.5 mm and 13.5 mm. At each of the positions, the counterfrequency variation of a reflection coefficient of high-frequency powerwas measured and displayed as the above case. The results are as shownwith curved lines Rb to Rf in FIG. 6.

The curved lines Ra to Rf show some absorption peaks Pa to Pd indicativeof strong absorption of the high-frequency plasma density information atthe plasma load side. The frequency in the absorption peaks Pa to Pd isplasma absorption frequency. It is possible to grasp the characteristicsof the generated plasma PM from the plasma absorption frequency.However, only the absorption peak Pa of the lowest frequency appears ina position of substantially constant frequency (1.5 GHz) even if the tipend length L is varied as shown in FIG. 7, and the same plasmaabsorption frequency is measured always. Namely, the plasma absorptionfrequency which does not depend on the tip end length L is plasmasurface wave resonance frequency f (=ω/2π). Even an absorption peakwhich appears at the lowest frequency side, if its frequency is variedwhen the tip end length L is varied, such an absorption peak is not theplasma surface wave resonance frequency. That is, in the presentembodiment, the tip end length L is varied so as to check whether theabsorption peak which appears at the lowest frequency side is plasmasurface wave resonance frequency.

If the plasma surface wave resonance frequency f is obtained, electronplasma angle frequency ω_(p) is obtained based on the above-mentionedexpression as follows:

ω_(p)=ω×{square root over ( )}(1+ε)=2π×1.5×10⁹×{square root over ()}(1+4)=3.35×10⁹

Further, electron density n_(e) of plasma PM is obtained as follows:

n _(e) =e _(o) ·m _(e)·ω_(p) /e=1.4×10¹¹/cm³

Since the electron density n_(e) of the plasma PM is substantiallyequivalent to the plasma density, it is easy to grasp (monitor) thecharacteristics of the generated plasma PM.

In the case of the present embodiment, since the tube is interposedbetween the plasma PM and the loop antenna 15 as well as the coaxialcable 16, a foreign object should not enter the plasma PM from the loopantenna 15 and the coaxial cable 16, and the clean state of the plasmacan be secured. Further, because the tube 14 is interposed, the loopantenna 15 and the coaxial cable 16 are prevented from being damaged bythe plasma PM. Furthermore, during the measurement, even if stainscomprising insulative films are thinly adhered on the surface of thetube 14, since the insulative film is dielectric, the measuring systemis not changed substantially, and variation is not caused in themeasured result due to the stains of insulative film. Therefore, it ispossible to measure the plasma density information over the long term.

Further, this method is carried out only by supplying the high-frequencypower from the loop antenna 15 through the tube 14 to grasp theabsorption phenomenon of the resonance high-frequency power which iseasily measured. Therefore, plasma density information can be measuredextremely easily. Furthermore, hot filament is not used, there is noneed to be worried about atmosphere contamination by evaporatedtungsten, and it is unnecessary to exchange the hot filament.

When it is necessary to measure plasma density information at anotherposition in plasma PM, the insertion length (shown with M in FIG. 1) ofthe measuring probe 12 into the chamber 1 may be changed to the otherposition, and the measurement may be carried out in the same manner asthat described above. By measuring the plasma densities at a pluralityof positions, it is possible to grasp the distribution of plasmadensity.

The present invention should not be limited to the above-describedembodiment, and can be modified and carried out as follows:

(1) Although the coaxial cable 16 provided at its tip end with the loopantenna 15 is covered with the dielectric tube 14 to form the measuringprobe 12 in the above embodiment, the measuring probe 12 is notnecessarily covered with the tube 14. That is, it is possible that acoaxial cable is formed at its tip end with the antenna such that theloop antenna 15 or the core wire 16 a projects like a needle, and thecoaxial cable is directly inserted to plasma to measure the plasmadensity. In this case, an insulative file may be adhered to the antennaexposed to plasma. However, in the present invention, high-frequencypower (electromagnetic wave) is radiated from the antenna, there islittle possibility that the measurement is influenced by the insulativefilm adhered to the antenna.

(2) In the above embodiment, the tube 14 of the measuring probe 12 isdetachably mounted to the wall of the chamber 1. However, this structuremay be modified such that the tube 14 of the measuring probe 12 ispreviously fixed to the wall of the chamber 1, and whenever themeasurement is carried out, the loop antenna 15, the cable 16 and theconductor piece 17 are inserted into the tube 14 to measure.

(3) According to the measuring method of the present invention, as shownin FIG. 6, some plasma absorption frequencies (absorption peaks Pb, Pc,Pd) are observed, other than the plasma surface wave resonance frequencyf corresponding to the absorption peak Pa. It is considered that theseother plasma absorption frequencies correspond to so-calledTonks-Dattner. As described above, the resonance frequency is related tothe electron plasma angle frequency ω_(p), if the plasma density ischanged, the Tonks-Dattner resonance frequency is also changed.Therefore, plasma density information can be obtained from theTonks-Dattner resonance frequency. However, since the plasma surfacewave resonance frequency f is directly related to the electron densityin plasma which is substantially equivalent to the plasma density, theplasma surface wave resonance frequency f is especially useful plasmadensity information.

(4) In the case of the above embodiment, the physical amount indicativeof absorption state of the frequency power by the plasma load was thereflection coefficient of the high-frequency power. In the presentinvention, impedance value of plasma load is also the physical amountindicative of absorption state of the frequency power by the plasmaload. In this case, the counter frequency characteristics of theimpedance of plasma load are measured using a channel analyzer.

(5) Although the single measuring probe 12 is disposed in the chamber 1in the case of the above embodiment, this structure can be modified suchthat a plurality of measuring probes 12 are disposed in the chamber 1.

(6) Although the plasma density information is obtained by inserting themeasuring probe 12 into plasma in the case of the above embodiment, itis not always necessary to dispose the measuring probe 12 into plasma.For example, the chamber 1 shown in FIG. 1 may be provided with adielectric window such as heat-resistant reinforce glass or quartz, anda frequency irradiation antenna may be provided outside the window, andhigh-frequency power may be irradiated to plasma in the chamber 1through this window.

(7) The shape and material of the measuring probe, and kind of theantenna of the invention should not be limited to those of the aboveembodiment. The plasma of interest of the present invention is not onlyplasma for processing, and includes plasma used for a particle beamsource or an analyzing apparatus.

(8) In the above embodiment, the length L between the base end of theloop antenna 15 and the tip end of the tube 14 in the measuring probe 12is varied, and plasma absorption frequencies of the same level areobtained at the various lengths as the plasma surface wave resonancefrequency f. This structure can be modified as follows. As shown in FIG.8, a plurality of wire-like antennas 15 a and 15 a as well as coaxialcables 16A and 16B are accommodated in the dielectric tube 14 a suchthat lengths La and Lb between the antenna base end and the tip end ofthe tube 14 are different. Then, the plasma absorption frequency isobtained for each of the antennas by the power reflection coefficientfrequency characteristic obtaining section 22 of the probe controlsection 13, and the plasma absorption frequency of the resonancefrequency is obtained as the plasma surface wave resonance frequency fby an absorption frequency comparator 22 a.

Alternatively, the wire-like antennas 15 a, 15 a and the coaxial cables16A, 16B may not be accommodated in a single dielectric tube, and may beseparately accommodated in different dielectric tubes 14, as shown inFIG. 9.

With these modifications, the plasma surface wave resonance frequency fcan be easily obtained even if the tip end length of the tube 14 is notvaried.

(9) In the case of the above embodiment, the reflection amount of thehigh-frequency power for measuring the plasma density information istaken out by the directional coupler 19. This structure can be modifiedsuch that the reflection amount of the high-frequency power is measuredby measuring the amount of electric current of high-frequency amplifierfor supplying high-frequency power for measuring the plasma densityinformation. The amount of electric current of the high-frequencyamplifier has extremely excellent correlation with the reflection amountof the high-frequency power, and it is easy to measure the amount ofelectric current.

More specifically, as shown in FIG. 10, the amount of electric currentof a high-frequency amplifier section 18 b provided next to a signaloscillator 18 a of the high-frequency oscillator 18 is taken out by anamplifier current detecting section 19 a, and sent to the powerreflection coefficient frequency characteristic obtaining section 22. Anexample of the amplifier current detecting section 19 a is a circuitstructure for detecting electric current value of a driving power sourceof the high-frequency amplifier section 18 b.

(10) As shown in FIG. 11, the above embodiment may be modified such thata power limiter 24 for stopping the high-frequency power which generatesexcessive plasma entering to the antenna 15 in the probe is providedbehind the plasma density information measuring probe 12. Especiallywhen the plasma PM disappears unexpectedly, there is an adversepossibility that the high-frequency power for generating the plasma isdirectly placed on the antenna 15, and the probe control section 13 isdestroyed. In this modification, the power limit 24 prevents theexcessive mixed high-frequency power from flowing into the probe controlsection, thereby preventing the probe control section 13 from beingdestroyed.

A switch (not shown) such as a relay type coaxial switch and asemiconductor type electronic switch may be used instead of the powerlimiter 24. The switch may carry out the on-off operation manually.However, in order to prevent the probe control section 13 from beingdestroyed, it is effective that the switch can detect that the reverselyflowing high-frequency exceeds a constant level (e.g., 1.2 times ofsupplied high-frequency power), and the switch is automatically turnedon or off, or plasma light is monitored by an optical sensor, and whenthe optical sensor detects that the plasma light disappear, the switchis automatically turned on or off.

(11) The above embodiment may be modified such that the measuring probe12 is inserted for forward and backward movement into the chamber inwhich the plasma PM is generated, and there is provided prove movingmeans for moving the measuring probe 12 such that the tip end of themeasuring probe 12 is pulled backward from the measuring position in thechamber 1 to a position close to the wall surface of the camber 1 whenthe measuring is not carried out. With this modification, even plasmagenerates stains thickly on the surface of the measuring probe 12, theamount of stains is reduced, and the lifetime of the measuring probe 12is elongated.

More specifically, as shown in FIG. 12, the measuring probe 12 isintegrally provided with a movable piece 25, the movable piece 25 isthreadedly engaged to a sending screw bar 26, and as a motor 27 rotatesand the sending screw bar 26 is rotated, the movable piece 25 isreciprocally moved in the longitudinal direction of the measuring probe12. When measuring is not carried out, the tip end of the measuringprobe 12 is pulled backward to a retreat position in the vicinity of thewall surface of the chamber 1 as shown with a solid line in FIG. 12, andwhen the measuring is carried out, the motor 27 is controlled such thatthe tip end of the measuring probe 12 is advanced to a measuringposition in the chamber 1 as shown with a dotted line in FIG. 12.

(12) The insulative material 16 c between the core wire 16 a and theshield wire 16 b of the coaxial cable 16 is fluorocarbon resin in theabove embodiment. FIG. 13 shows a modification in which an insulativematerial 16 d filling a gap between the core wire 16 a and a conductortube 16 e for shield is heat-resistant (insulative) ceramics. In thiscase, the heat-resistance of the coaxial cable 16 is enhanced.

(13) In a measuring probe 12 of another embodiment shown in FIG. 14, adielectric tube 14 is covered with a metal film 28 such that only ameasuring region is not covered with the metal film 28. That is, aportion of the metal film 28 corresponding to the measuring region iscut out to form a window 28 a. The high-frequency power does not enterthe metal film 28 and can only enter the window 28 a. Therefore, a localstate of the measuring area that is not coated with the metal film 28 isstrongly reflected to the measuring result and as a result, the spatialresolution can be enhanced.

(14) In a measuring probe 12 of another embodiment shown in FIG. 15, theloop antenna 15 is replaced by a wire antenna 15 a, and the wire antenna15 a is extended closely along the inner surface of the dielectric tube14. With this structure, there are merits that the high-frequency poweris efficiently supplied, and the required high-frequency power can bereduced, and the measuring precision is enhanced. Even with the loopantenna 15, if it is extended closely along the inner surface of thedielectric tube 14, the high-frequency power is likewise suppliedefficiently.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

What is claimed is:
 1. A probe used for measuring plasma densityinformation, comprising: a dielectric tube whose tip end is closed; anantenna accommodated in said tube at its tip end side for radiatinghigh-frequency power; and a cable accommodated in said tube at its rearside and connected to said antenna for transmitting said high-frequencypower.
 2. A probe used for measuring plasma density informationaccording to claim 1, wherein said antenna and said cable accommodatedin said dielectric tube are capable of moving along a longitudinaldirection of said tube such that a position of said antenna in said tubecan be varied.
 3. A probe used for measuring plasma density informationaccording to claim 1, wherein a conductor for preventing a leakage ofejected electromagnetic wave from said antenna is disposed at a positionslightly back from said antenna such as to occlude a gap between saidcable and an inner surface of said tube.
 4. A probe used for measuringplasma density information according to claim 1, further comprisingprobe cooling means for forcibly cooling said probe.
 5. A probe used formeasuring plasma density information according to claim 1, wherein saidcable for transmitting high-frequency power comprises a conductor tubefor a core wire and a shield, and an insulative ceramics material forfilling a gap between said core wire and said conductor tube.
 6. A probeused for measuring plasma density information according to claim 1,wherein a surface of said dielectric tube is coated with metal such thata measuring area of said dielectric tube is not coated.
 7. A probe usedfor measuring plasma density information according to claim 1, whereinsaid antenna is extended closely along an inner surface of saiddielectric tube.
 8. A plasma density information measuring apparatus,comprising: sweep-frequency type high-frequency power supplying meansfor supplying high-frequency power to plasma while sweeping frequency;reflection power amount detecting means for detecting a reflectionamount of said high-frequency power; and power reflection coefficientfrequency characteristics obtaining means for obtaining a counterfrequency variation of reflection coefficient of high-frequency powerbased on a sweep-frequency of said high-frequency power and the detectedresult of said reflection amount of high-frequency power.
 9. A plasmadensity information measuring apparatus according to claim 8, furthercomprising a dielectric division wall interposed between plasma and saidsweep-frequency type high-frequency power supplying means.
 10. A plasmadensity information measuring apparatus according to claim 9, furthercomprising a dielectric tube whose tip end is closed, an antennaaccommodated in said tube at its tip end side for radiatinghigh-frequency power, and a cable accommodated in said tube at its rearside and connected to said antenna for transmitting said high-frequencypower, wherein high-frequency power is supplied from said antenna insaid tube to plasma using a tube wall of said dielectric tube as adivision wall, a plurality of antennas are accommodated in saiddielectric tube such that distances between a tip end of said tube andsaid antennas are different from one another, and said power reflectioncoefficient frequency characteristics obtaining means obtains a counterfrequency variation of reflection coefficient of high-frequency powerfor each of said antennas, and a plasma absorption frequency appearingat the same frequency in the counter frequency variations is obtained asa plasma surface wave resonance frequency.
 11. A plasma densityinformation measuring apparatus according to claim 10, wherein a plasmadensity information measuring probe is inserted in a chamber whichgenerates plasma for forward and backward movement, and said probe ismoved such that a tip end of said probe is pulled backward from ameasuring position in said chamber to a retreat position in the vicinityof a wall surface of said chamber when measurement is not carried out.12. A plasma density information measuring apparatus according to claim10, wherein protecting means for blocking excessive plasma generatinghigh-frequency power which enters said antenna in said probe is providedbehind said plasma density information measuring probe.